[ty] Implement long-polling for workspace diagnsotics (#19670)

crates/ruff_server/src/server/schedule/task.rs

@@ -33,6 +33,7 @@ pub(in crate::server) enum BackgroundSchedule {
 /// while local tasks have exclusive access and can modify it as they please. Keep in mind that
 /// local tasks will **block** the main event loop, so only use local tasks if you **need**
 /// mutable state access or you need the absolute lowest latency possible.
+#[must_use]
 pub(in crate::server) enum Task {
     Background(BackgroundTaskBuilder),
     Sync(SyncTask),